1. Field of the Invention
The present invention relates to a wire bonding apparatus and method used in a manufacturing process for semiconductor devices.
2. Description of the Related Art
FIG. 17 shows a conventional wire bonding apparatus. A capillary tip 1 is supported by an ultrasonic horn 2. A first clamper 3 is disposed above the capillary tip 1, and is supported by a first clamper switch 4 which is arranged integrally with the ultrasonic horn 2. The ultrasonic horn 2 and the first clamper switch 4 are coupled to an up/down motion mechanism 5. That is, the capillary tip 1 and the first clamper 3 are moved up and down by the up/down motion mechanism 5 while the relative positions of the above two components are kept constant. A second clamper 6 is supported by a second clamper switch 7 and positioned above the first clamper 3. This second clamper 6 does not move up and down as it is secured at a fixed position. The first and second clampers 3 and 6 clamp a fine metal wire 8 which passes through the capillary tip 1.
A bonding operation will now be explained with reference to FIGS. 18 through 23. First, the capillary tip 1 is used to join a ball 11 formed at the tip of the fine metal wire 8 to a bonding pad of an IC chip 10 on a die pad 9 (FIG. 18). Next, the capillary tip 1 is moved upward (FIG. 19) and over an inner lead 12 while letting out the fine metal wire 8 (FIG. 20). It is then moved downward to join the fine metal wire 8 to the inner lead 12 (FIG. 21). Thereafter, while the first clamper 3 continues to clamp the fine metal wire 8, it is moved upward together with the capillary tip 1, whereby the fine metal wire 8 is cut (FIG. 22). Finally, the tip of the fine metal wire 8 sticking out of the capillary tip 1 is formed into another ball 11 (FIG. 23). This completes one cycle of a bonding process.
The operations performed by the first and second clampers 3 and 6 when the fine metal wire 8 is joined to the inner lead 12, as shown in FIGS. 20 and 21, will be explained below with reference to FIGS. 24 through 29. First, the first clamper switch 4 closes the first clamper 3 which in turn clamps the fine metal wire 8 (FIG. 24). Under this condition, the up/down motion mechanism 5 moves the first clamper 3 as well as the capillary tip 1 downward (FIG. 25). When the capillary tip 1 approaches the inner lead 12, the first clamper 3 is opened (FIG. 26). While the first clamper 3 is opened, the capillary tip 1 and the first clamper 3 are moved a little further downward (FIG. 27). Next, the second clamper switch 7 closes the second clamper 6 which in turn clamps the fine metal wire 8 (FIG. 28). The capillary tip 1 and the first clamper 3 are then moved downward to join the fine metal wire 8 to the inner lead 12 (FIG. 29).
As shown in FIGS. 24 and 25, however, the first clamper 3 clamps the fine metal wire 8 above the inner lead 12. While it continues to clamp the fine metal wire 8, the first clamper 3 and the capillary tip 1 are moved downward. Therefore, as shown in FIG. 28, by the time the capillary tip 1 almost comes in contact with the inner lead 12, the excess fine metal wire 8 has been drawn under the capillary tip 1.
As illustrated in FIGS. 30 and 31, for the above reason, the bottom dead center A of the fine metal wire 8 under the capillary tip 1 is positioned away from the center axis 0 of the capillary tip 1. Therefore, as shown in FIGS. 32 and 33, as the capillary tip 1 is moved downward, the fine metal wire 8 does not come in contact with the inner lead 12 at the bottom dead center A, but instead it comes in contact with the inner lead 12 at a point B which is closer to the capillary tip 1 than the bottom dead center A is. As depicted in FIGS. 34 and 35, when the capillary tip 1 is completely in contact with the inner lead 12, the fine metal wire 8 slips off the inner lead 12 at the point B. As a result, the fine metal wire 8 bends and hangs appreciably. When such a problem occurs, the fine metal wire 8 comes in contact with adjacent wires or inner leads, which may cause a short circuit.